Honda Civic Owners Manual: Treadwear

The treadwear grade is a comparative rating based on the wear
rate of the tire when tested under controlled conditions on a
specified government test course. For example, a tire graded 150
would wear one and one-half (1 1/2) times as well on the
government course as a tire graded 100. The relative performance
of tires depends upon the actual conditions of their use, however,
and may depart significantly from the norm due to variations in
driving habits, service practices and differences in road
characteristics and climate.